Method of producing rough polysilicon by the use of pulsed plasma chemical vapor deposition and products produced by the same

ABSTRACT

A method for depositing a rough polysilicon film on a substrate is disclosed. The method includes introducing the reactant gases argon and silane into a deposition chamber and enabling and disabling a plasma at various times during the deposition process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to the field of integratedcircuit manufacturing and, more particularly, to a method for forming arough polysilicon film.

[0003] 2. Description of the Related Art

[0004] In semiconductor manufacturing, it is sometimes desirable todeposit polysilicon film on a substrate in such a way that roughpolysilicon results. In dynamic random access memory (DRAM)technologies, for instance, it may be desirable to manufacture a memorycell capacitor using rough polysilicon.

[0005] A capacitor having a given lateral area manufactured with a roughpolysilicon plate will have a higher capacitance than a capacitor ofhaving the same lateral area manufactured with a smooth plate. The roughpolysilicon capacitor has a larger effective dielectric surface area dueto the folding of the capacitor dielectric over the rough film. Whenused in a DRAM memory cell, the larger dielectric surface area increasesthe memory cell's capacitance and improves the cell's charge storagecharacteristics, thereby improving product performance.

[0006] Unfortunately, the use of traditional manufacturing methods toproduce rough polysilicon have been generally unsatisfactory because ofthe widely varying roughness they produce. The instant process resultsin a more reliable rough polysilicon film that is more easilycontrolled. Moreover, because the instant process uses etchants andtechniques common to the semiconductor industry, this improved processcan be readily integrated into modern day manufacturing flows.

SUMMARY OF THE INVENTION

[0007] In accordance with one aspect of the present invention, there isprovided a method for depositing a rough polysilicon film on asubstrate. The method includes the steps of introducing reactant gasinto a deposition chamber containing the substrate, and enabling anddisabling a plasma within the deposition chamber.

[0008] In accordance with another aspect of the present invention, thereis provided a method for manufacturing a capacitor having a plate formedof rough polysilicon. The plate is formed by introducing reactant gasinto a deposition chamber containing the substrate and selectively andrepeatedly enabling and disabling a plasma to deposit the roughpolysilicon on the substrate. A dielectric layer is formed on the roughpolysilicon, and a conductive plate is formed on the dielectric layer.

[0009] In accordance with a further aspect of the present invention,there is provided a capacitor that includes a polysilicon plate formedby introducing reactant gases into a deposition chamber and selectivelyand repeatedly enabling and disabling a plasma at various times duringthe deposition process. A dielectric film is disposed on the polysiliconplate, and a top plate is disposed on the dielectric film.

[0010] In accordance with yet another aspect of the present invention,there is provided a memory circuit that includes a plurality of memorycells. Each of the memory cells has a capacitor. Each capacitor has aplate of rough polysilicon formed by introducing reactant gas into adeposition chamber and selectively and repeatedly enabling and disablinga plasma at various times during the deposition process.

[0011] In accordance with a still further aspect of the presentinvention, there is provided a method for depositing a rough polysiliconfilm on a substrate. The method includes the steps of: (a) placing asubstrate in a deposition chamber; (b)introducing reactant gas into saiddeposition chamber; (c) creating a plasma; (d) disabling said plasma;(e) pumping a portion of reactive by-products out of said depositionchamber; and (f) repeating steps (c), (d), and (e) until a roughenedpolysilicon film has been deposited on said substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing and other advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0013]FIG. 1A is a cross-section of a bottom plate of a capacitorcomprised of rough polysilicon.

[0014]FIG. 1B shows the addition of a capacitor dielectric over therough polysilicon film.

[0015]FIG. 1C shows the addition of a top plate of a capacitor over thecapacitor dielectric.

[0016]FIG. 2 is a stylized representation of a polysilicon depositionchamber.

[0017] While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0018] Turning now to the drawings, the formation of a capacitor,generally designated by a reference numeral 10, using a roughpolysilicon is shown in cross-section in FIGS. 1A through 1C. In FIG.1A, a rough polysilicon film 12 is formed over a substrate 14. The roughpolysilicon film 12 constitutes the bottom plate of the capacitor 10. InFIG. 1B, a dielectric 16 is grown or deposited on top of the roughpolysilicon film 2. In FIG. 1C, a layer of conductive material 16 (e.g.,polysilicon) is deposited over the dielectric 16. The conductivematerial 18 constitutes the top plate of the capacitor 10. In oneembodiment, pulsed plasma chemical vapor deposition (CVD), using amixture of silane gas (SiH₄) and argon (Ar), is used to deposit therough polysilicon film 12 on the underlying substrate 14.

[0019] A stylized representation of a deposition chamber 20, which canbe used to practice the disclosed process, is shown in FIG. 2. Thesubstrate 14 is shown positioned in the deposition chamber 20 atop awafer chuck 22. A heating element 24 runs through the wafer chuck 22.The temperature of the heating element 24 may be varied by a temperaturecontroller 26. An RF power source 20 sends an oscillating voltage at acontrolled frequency to a plate 30 placed above the wafer chuck 22. Theplate 30 is usually grounded to create a radio-frequency (RF) controlledplasma. The voltage that builds up between the plate 30 and the waferchuck 22 creates an electric field inside of the deposition chamber 20.When this happens, gases that have been introduced into the chamber viaa port 32 and a port 34 can become ionized so that the reactants aredeposited onto the substrate 14. This is commonly referred to as“striking” or “enabling” a plasma.

[0020] More than just two gases can be introduced into the depositionchamber 20 at one time if the reaction chemistry so dictates, as one ofordinary skill will realize. A suitable deposition chamber 20 forpracticing the disclosed embodiment is an Applied Materials™ model 5000Ddeposition chamber. This model is advantageous because it containsplasma generation capabilities. Other more standard CVD chambers, suchas the Applied Materials™ Centura™ model 5200 or the AG Associates™model 8000, may also be used if modified to generate a sufficientelectric field to enable a plasma.

[0021] In the disclosed embodiment, the plasma is “pulsed,” meaning thatthe plasma is selectively enabled and disabled at various times duringdeposition. Pulsing of the plasma can be achieved by several means. Onesuch method involves connecting a pulse generator 36 to the RF powersource 28, as shown schematically in FIG. 2. The pulse generator 36sends a square wave to the RF power source 28. The square wave isconnected to the RF power source 28 in such a manner that the RF powersource 28 is enabled during the high portions of the square wave 28 andis disabled during the low portions of the square wave. When the RFpower source 28 is enabled, it sends an oscillating voltage to the plate30 as described above, thus facilitating formation of plasma in thedeposition chamber 20. When the RF power source 28 is disabled, theoscillating voltage is not sent to the plate 30. Thus, no plasma isformed during that period. It is also possible that the RF power source28 can be enabled and disabled by the use of a computer or by the use ofanother appropriate switch.

[0022] For sufficient production of a rough polysilicon film, the RFpower source 28, in this embodiment, is capable of producing a power inthe range of 10 to 1,000 Watts and oscillates (when enabled) with afrequency of 400,000 to 15,000,000 Hertz (Hz), with 13,600,000 Hz beingtypical. The pulse generator 36 is capable of producing a square wavewith a frequency of 25 to 1,000 Hz. The duty cycle, i.e., the percentageof the time that the square wave is high, may vary between 10% and 90%.Assuming a six-liter volume for deposition chamber 20, the followingother deposition parameters are suitable for the production of a roughpolysilicon film: deposition temperature of 300 to 600 degreesCentigrade; silane gas flow rate of 5 to 500 cubic centimeters perminute; argon gas flow rate of 10 to 1000 cubic centimeters per minute;and deposition chamber pressure of 0.1 to 100 Torr.

[0023] The pulsing of the plasma has distinct advantages. First, pulsingthe plasma allows for stringent control of the plasma chemistry. Duringthe disabled portion of the plasma cycle, the reactive by-products arepumped out of the deposition chamber 20 by the pump 40, and the reactionis started anew on the next enabled cycle. If the plasma is not pulsed,the silane ions produced in the plasma will combine to form siliconwhich, when situated on the wafer surface, may produce a smooth film.

[0024] Another advantage of pulsing the plasma is that gas phasenucleation is generally not conducive to precise control, becausenucleation size and density vary as a function of deposition time in anactive plasma. By keeping the enabled time relatively short, theseeffects, which tend to cause the polysilicon film to grow in a randomand unrepeatable fashion, are reduced. With a pulsed plasma, a constantdesired concentration of reactive silane can be maintained becauseduring the disabled state, the silane ions become neutral throughrecombination.

[0025] The disclosed process has other advantages as well. First,because the mean free path of the reactive species is kept to a maximumby curtailing the duration of the “enabled” portion, better polysiliconstep coverage can result. This makes the disclosed process beneficialfor depositing a rough polysilicon film on a three-dimensional surface,such as a contact hole, a trench, or a stack for a DRAM capacitor. Inaddition, rough polysilicon films using the disclosed technique can bedeposited at relatively low temperatures, such as 500 degreesCentigrade. Deposition at low temperatures is beneficial because thegrowth of the polysilicon film on the surface of the wafer will not bedictated by the thermal mobility of the silicon atoms which dissociatefrom the silane ions at the wafer surface. By the use of lowerdeposition temperatures, a rough polysilicon film can be achieved thatis largely independent of temperature. By contrast, conventionalnon-pulsed CVD rough polysilicon films cannot typically be deposited atsuch low temperatures, because the resulting film will be amorphous and,therefore, smooth.

[0026] The use of the disclosed method produces a film in which thevertical “peak-to-valley” roughness of the polysilicon film isapproximately 500 Angstroms, and the lateral “peak-to-valley” roughnessof the polysilicon film is approximately 500 Angstroms. However, theresulting film is a function of the various processing conditions thatare discussed herein, and the process is susceptible to optimizationbefore a film of a suitable rugosity is realized for a givenapplication.

What is claimed is:
 1. A method for depositing a rough polysilicon filmon a substrate, said method comprising the steps of: (a) introducingreactant gas into a deposition chamber containing said substrate; and(b) enabling and disabling a plasma within the deposition chamber. 2.The method of claim 1 , wherein step (a) comprises the step ofintroducing argon and silane into said deposition chamber.
 3. The methodof claim 1 , wherein step (b) is performed at a temperature below 500degrees Centigrade.
 4. The method of claim 1 , wherein step (b)comprises the step of selectively enabling and disabling said plasmausing a function generator.
 5. A method for manufacturing a capacitorhaving a plate formed of rough polysilicon, said method comprising thesteps of: (a) forming the plate by introducing reactant gas into adeposition chamber containing said substrate and selectively andrepeatedly enabling and disabling a plasma to deposit said roughpolysilicon on said substrate; (b) forming a dielectric layer on saidrough polysilicon; and (c) forming a conductive plate on said dielectriclayer.
 6. The method of claim 5 , wherein step (a) comprises the step ofintroducing argon and silane into said deposition chamber.
 7. The methodof claim 5 , wherein step (b) is performed at a temperature below 500degrees Centigrade.
 8. The method of claim 5 , wherein step (b)comprises the step of selectively enabling and disabling said plasmausing a function generator.
 9. A capacitor, comprising: (a) a bottompolysilicon plate which is formed by introducing reactant gases into adeposition chamber and selectively and repeatedly enabling and disablinga plasma at various times during the deposition process; (b) adielectric film disposed on the bottom polysilicon plate; and (c) a topplate disposed on the dielectric film.
 10. The capacitor of claim 9 ,wherein the introduction of reactant gases includes the introduction ofargon and silane.
 11. The capacitor of claim 9 , wherein the bottompolysilicon plate is formed at a temperature below 500 degreesCentigrade.
 12. The capacitor of claim 9 , wherein the selective andrepeated enabling and disabling of the plasma is accomplished by afunction generator.
 13. A memory circuit comprising: a plurality ofmemory cells, each of said plurality of memory cells having a capacitor,each capacitor having a plate of rough polysilicon formed by introducingreactant gas into a deposition chamber and selectively and repeatedlyenabling and disabling a plasma at various times during the depositionprocess.
 14. The memory circuit of claim 13 , wherein the majority ofplates of rough polysilicon have a vertical roughness of about 500Angstroms and a lateral roughness of about 500 Angstroms.
 15. A methodfor depositing a rough polysilicon film a substrate, said methodcomprising the steps of: (a) placing a substrate in a depositionchamber; (b) introducing reactant gas into said deposition chamber; (c)creating a plasma; (d) disabling said plasma; (e) pumping a portion ofreactive by-products out of said deposition chamber; and (f) repeatingsteps (c), (d), and (e) until a roughened polysilicon film has beendeposited on said substrate.